1. Field of the Invention
The present invention relates to a method for producing potato microtubers whereby large microtubers which can be directly planted in the field are efficiently produced.
2. Description of Related Art
A first method for producing potato microtubers that is popularly employed in practice comprises the steps of growing virus-free plants and then forming and growing microtubers.
A second method is described in Japanese Laid-open Patent Application No. 3-35738. This Laid-open Patent Application discloses a process for producing potato microtubers comprising the following three steps:
(A) Potato plants are tissue-cultured to produce virus-free plants having stems and leaves (Illumination: 1,000-100,000 lux for 6-24 hours/day, at 20.degree.-30.degree. C.); PA1 (B) The plants are cultured at a low temperature (10.degree.-30.degree. C.) under a short daylength (1,000-100,000 lux for 12 hours or less), thereby inducing a state in which the apical buds or axillary buds are converted to microtubers; PA1 (C) Apical buds or axillary buds induced in the preceding step are converted into microtubers in the dark at a low temperature (10.degree.-30.degree. C.).
In the second method, a step of inducing the state in which the apical buds and axillary buds are converted to microtubers at low temperature and under a short daylength is inserted between the two steps of the first method. This step was presumably inserted taking into consideration the general property of potatoes that the formation of microtubers is induced at a low temperature and under a short daylength, which property is known and described in the literature.
Thus, via the second method, microtubers are effectively produced by three steps, i.e., step A: growth of virus-free plants; step B: induction of the state in which the apical buds and axillary buds are converted into microtubers; and step C: formation and growth of microtubers.
It is another characteristic feature of the second method that step C is carried out at a low temperature in the dark. In general, the formation and growth of the microtubers are carried out in the dark or under a short daylength. Although the reason why the low temperature condition is indispensable is not clear, it is probably because the formation of potato microtubers is promoted at low temperature.
It seems true that the conversion into microtubers is promoted, and the number of formed microtubers is increased, by the short day treatment after the stem and leaf-growing step. However, since the efficiency of production of large microtubers (not less than 0.5 g) which can be directly planted in the field is low due to the following reasons, this method is not practical.
In the second method, subsequent to the plant-growing step, microtubers are induced at a low temperature and under a short daylength using the same medium as used in the plant-growing step. This medium has a low sugar content, and immediately after exchanging the medium with one having a high sugar content, the potato plants are cultured in the dark to form microtubers. Since the potato plants are cultured on a medium having a low sugar content at a low temperature under a short daylength at the stage in which the growth of stems and leaves is active, the growth of plants is not good, and even if the number of total microtubers is increased, the number of large microtubers is very small.
Since the growth of plants and the induction of the converting state into microtubers are carried out using an agar medium in a small vessel (0.3-0.6 liter), the efficiency of growth of plants is lower than in the case where culturing is carried out by liquid aeration culture using a large vessel. Thus, the practical production of large microtubers cannot be attained by this method.
In Japanese Laid-open patent application (Kokai) No. 3-35738 (second method), the conditions of steps B and C are "at a low temperature and under short daylength" and "in the dark and at low temperature," respectively. Thus, in both steps, low temperature is indispensable. However, the temperature conditions employed in two examples are ordinary temperature conditions which are generally employed in the tissue culture of potatoes. The temperature conditions are also within the known range employed in the production of microtubers.